It sounds like you had a USB ground loop, see https://duet3d.dozuki.com/Wiki/USB_ground_loops. Check that the wire between the ground side of the Duet VIN terminal block and your PSU is secure. In particular, check that the screws in the VIN terminal block are still tight.

If you have a non-integrated 7" PanelDue in your system, then don't expect to be able to power it from USB unless you disconnect the PanelDue first. Otherwise, try a new USB cable. If that doesn't fix the problem then the ground connector on the USB socket is probably damaged. The socket can be replaced, using either hot air rework equipment or low melting point solder such as ChipQuik to remove the old one.

Yeh I always use my laptop without having it hooked up to mains for this reason but since the laptop was not around I just hooked up my desktop without thinking... I think the pin closest to the SD card (gnd??) is burned out. I thought at first that it was the usb cable that burnt out so I replaced that but now when I look closer I think that pin is missing. Is it possible to get it back alive without replacing the micro usb port? I don't really need it since I just use it when FW update fails ^^

Cut the micro USB plug off the end of the cable. Solder the data wires to the ends of R2 and R3 where the traces from the USB connector went (it does matter which way round). Solder the ground wire to a ground point, such as the ground side of the Reset switch. You can leave the 5V wire not connected unless you want to be able to power the Duet from USB.

Buy a USB-to-serial converter with 3.3V signal levels, and update the firmware through the PanelDue port instead.

Ok. I do have serial converters lying around I think. I'll check later this evening. Is it D- on R3, D+ on R2 and vcc on R1? Kinda hard to track the traces when a moron have messed them up like this

Cut the micro USB plug off the end of the cable. Solder the data wires to the ends of R2 and R3 where the traces from the USB connector went (it does matter which way round). Solder the ground wire to a ground point, such as the ground side of the Reset switch. You can leave the 5V wire not connected unless you want to be able to power the Duet from USB.

Buy a USB-to-serial converter with 3.3V signal levels, and update the firmware through the PanelDue port instead. I haven't tried this, but it should work. The same considerations about USB ground loops will still apply.

Cut the micro USB plug off the end of the cable. Solder the data wires to the ends of R2 and R3 where the traces from the USB connector went (it does matter which way round). Solder the ground wire to a ground point, such as the ground side of the Reset switch. You can leave the 5V wire not connected unless you want to be able to power the Duet from USB.

Buy a USB-to-serial converter with 3.3V signal levels, and update the firmware through the PanelDue port instead. I haven't tried this, but it should work. The same considerations about USB ground loops will still apply.

It just says "connecting" in proterface with a usb to serial converter powering the board. It has 3.3V serial level and I've hooked it up with RX->UTXD0, TX->URXD0, gnd->gnd and +5v -> +5v. It has a light on the 5v and the 3.3v led. Doesn't want to connect properly

If I try to solder on a usb pigtail to where the usb port was it just says unknown USB device (device descriptor request failed). I'm pretty sure I got D+/- in the correct orientation. I measure approx 3V from V- to R2 when USB is connected and the board is powered up.

Could I have hit reset or erase by accident and wiped the board so I need to flash it with another method?

To answer myself on the pinout.. DDM is D- and DDP is D+. I had them connected the wrong way. When I turned it the proper way around it connects without problems but it's throwing a lot of errors in pronterface with "Received opcode 000013e lenght 0 data" and crashes pronterface. I guess I need to find a way to flash it but now USB is running atleast.

Got it sorted. Had to disable the wifi module, update the firmware from scratch and add a new setnetwork file to connect again.

I would not expect Pronterface to work with the serial port unless you patch it to not expect an "ok" response to every command. RRF sends that response to commands received on the USB port when Marlin compatibility mode is selected, but not on the serial port. But you should be able to flash firmware via the serial port.